Optical fiber is becoming the transmission medium of choice for communication networks. The bandwidth of well designed optical fiber is measured in the hundreds of terahertz, and the system capacity is limited not by the fiber but by the electronics at its ends. Fiber's attenuation can be reduced to a level allowing transmission over hundreds of kilometers without regeneration or amplification. Fiber further is largely immune to electronic noise.
Optical fiber was originally used as a point-to-point replacement for electrical cable, such as coaxial cable. In this architecture, at the transmitter end of the fiber, a data signal modulates a laser emitting its light of a well defined wavelength into the fiber. At the receiver end of the fiber, a detector detects the intensity envelope of the light, thus converting the transmission signal back from the optical domain to the electrical domain. It was early recognized that the fiber capacity could be significantly increased by wavelength-division multiplexing (WDM). A plurality W of lasers of differing emission wavelengths are modulated by separate data signals, and their outputs are combined (multiplexed) onto a single fiber. At the receiver end, an optical demultiplexer separates the W-wavelength signal into W separate optical paths according to wavelength. A detector is associated with each optical path so that the W detectors output W electrical signals. However, the described WDM optical transmission system without additional specialized elements is a point-to-point system having optical fibers connecting two nodes with an opto-electronic conversion being required at each node. If an optical signal is to be transmitted through the node, the node may act as a regenerator in detecting the received optical signal, converting it to an electrical signal, and using the electrical signal to generate a new optical signal for transmission, and the regeneration is required even if the optical signal will transit the node without change.
The principle advantage of fiber is its low-cost bandwidth. However, multiplexers, demultiplexers, opto-electronic converters, and high-speed electronics associated therewith are expensive, and regenerators are replete with such elements. Further, the electronic design of regenerators typically depends strongly upon the format of the signal and its bit rate. As a result, any upgrade in data rate or conversion to a different signal type requires significant changes at each node of the network, thus greatly increasing the cost of any such change.
Brackett et al. have suggested an all-optical network to solve some of these problems, as described in "A scalable multiwavelength multihop optical network: A proposal for research on all-optical networks," Journal of Lightwave Technology, vol. 11, no. 5/6, 1993, pp. 736-753. In one type of an all-optical network 10, illustrated in the network diagram of FIG. 1, a number of nodes 12, designated respectively as A, B, C, D, E, transmit and receive respective multi-wavelength WDM signals onto and from the network 10. Three wavelengths .lambda..sub.1, .lambda..sub.2 .lambda..sub.3 are illustrated, but the number W of wavelengths may vary both between networks and over time on a single network 10. The network 10 includes a web of optical fibers 14 between wavelength-selective cross-connects (WSXCs) 16 and between the wavelength-selective cross-connects 16 and the nodes 12. The figure illustrates the WDM paths, not the fibers. Ignoring complexities like anti-parallel fibers for bidirectional transmission, very high-capacity links, and multi-fiber self-healing networks, two or more WDM signals are assumed to be carried in one direction between nodes 12 and WSXCs 16 on a single fiber. Importantly, the wavelength-selective cross-connects 16 can receive a W-wavelength WDM signal and switch its wavelength components in different directions without the need for converting the WDM optical signal to electrical form. For example, the A node 12 can transmit two signals of wavelengths .lambda..sub.1, .lambda..sub.3 over a single fiber 14. The wavelength-selective cross-connects 16 can switch the two wavelength signals at .lambda..sub.1, .lambda..sub.2 separately to the B and E nodes 12 according to the wavelength. That is, the switching is all-optical, and the opto-electronic conversion is confined to the nodes 12, not to the network 10 itself. This WDM network 10 can be characterized as transparent in the sense that an uninterrupted optical path exists between the transmitting and receiving nodes.
To preserve non-blocking transmission capability between nodes, the number W of WDM wavelengths needs to increase with the number of nodes 12. However, this number W seems to be limited to a relatively small number because of the need to optically amplify the optical signals (the favored erbium-doped fiber amplifier has a limited flat-gain band) and because of the limited bandwidth of many of the preferred wavelength-selective cross-connects. Systems are being demonstrated with W equal to four. This number is planned to be increased to sixteen or twenty. The all-optical network as described, however, is inadequate for interlinking a substantially larger number of nodes. Since the required number of wavelengths grows with the number of nodes, such an architecture is not scalable to a significantly larger network size.
As recognized by Brackett et al. ibid. and by Bala et al. in "The case for opaque multiwavelength optical networks," 1995 Digest of the LEOS Summer Topical Meetings, Keystone, Colo., Aug. 7-11, 1995, pp. 58, 59, the number of interlinked nodes can be increased by wavelength reuse and wavelength conversion. The network of FIG. 1 shows reuse of the wavelength .lambda..sub.1 in that the same wavelength is used between nodes AS and B and between nodes C and D. Wavelength reuse within a single network 10 can be extended if a node 12 can receive a data signal from another node at one wavelength and transmit that same data signal to yet another node at a second wavelength. This process is generally referred to as wavelength interchange or conversion. However, a more straightforward application of wavelength conversion occurs at the cross connect between two WDM networks.
As illustrated in the network diagram of FIG. 2, two all-optical networks 10.sub.1, 10.sub.2 are connected by a wavelength-interchanging cross connect (WIXC) 20. Only two networks are shown, but the concept scales to a large number of networks interconnecting a nearly arbitrarily large number of nodes 12. It is assumed that enough WDM wavelengths are available within each all-optical network 10.sub.1, 10.sub.2 to provide the wavelength-identified links between the nodes 12, including the wavelength-interchanging cross-connect 20, so that wavelength-selective switching suffices within each network 10.sub.1, 10.sub.2. On the other hand, it is likely that the number of WDM wavelengths is insufficient to provide the required number of such wavelength-identified links between the nodes 12 of both networks 10.sub.1, 10.sub.2.
The wavelength-interchanging cross-connect 20 alleviates this problem of insufficient number of WDM wavelengths with its capability of receiving a WDM signal from the first network 10.sub.1 at a first wavelength .lambda..sub.1 and retransmitting it onto the second network 10.sub.2 at another wavelength .lambda..sub.j.
The network in FIG. 1 can be characterized as implementing the architecture of a mesh network having a relatively large number of switching nodes 16 (more than the two illustrated) within the all-optical network 10 and being intra-connected within the network 10 by an irregular mesh of fibers. Each WSXC 16 may directly connect to multiple nodes 12 and to a number of other WSXCs 16 depending upon the network connectivity.
Another type of network architecture for fiber-based networks is a ring architecture which provides a high degree of survivability in the event of a break in the fiber or a failure of a switching node. In a ring network, the WSXC 16 is characterized as an add/drop multiplexer (ADM) associated with one node 12, and it is further connected to two neighboring ADMs on the ring.
An inter-connected two-ring network is illustrated in the network diagram of FIG. 3. Each of two rings 22.sub.1, 22.sub.2 includes a pair of counter rotating fibers 24, 26, and nodes 28 are serially arranged around the rings 22.sub.1, 22.sub.2 and connected to both the fibers 24, 26. A wavelength-interchanging cross-connect 29 is connected to all the fibers 24, 26 of both rings 22.sub.1, 22.sub.2 and serves to interconnect the rings with the additional capability of wavelength conversion as required. In the context of present-day telephony, the nodes 28 as well as the WIXC 29 are likely to be telephone central offices having additional input and output lines connecting the central offices to the local network. At least at the present time, switching between the rings and the local network will involve the central office converting the optical signal to an electrical signal and subsequently electrically switching the local traffic. A principal advantage of the ring architecture is that if the paired fibers 24, 26 are broken at one spot, for instance, in a construction accident, the signals on the ring can be rerouted to the fiber rotating in the other direction so as to maintain fall connectivity. Even if one node 28 fails, traffic can be rerouted so as to avoid that node, thus providing full connectivity between the remaining nodes.
It should be apparent that the WIXC 29 for the ring architecture provides much the same functions as the WIXC 20 for the mesh architecture. The ring architecture also emphasizes that the ring nodes 28 are operating as add-drop multiplexers (ADMs). In so far as the switching nodes 16 of the mesh architecture are located at central offices, they too can be explained in terms of ADMs.
An add/drop multiplexer (ADM) is a fundamental element in most communication networks using multiplexing on a single physical channel. The ADM is connected to a transmission path and is capable of extracting (dropping) one of the multiplexed signals from the path and further capable of inserting (adding) a signal to the optical path in place of the dropped signal. In some sense, the remaining multiplexed signals are not affected by the add/drop operation. For optical wavelength-vision multiplexed system, several fundamental multi-wavelength add/drop multiplexers (WADMs) are available which can switch one or more selected wavelengths into and out of an optical path without an opto-electronic conversion. Such WADMs may be implemented by the acousto-optical filter described by Cheung et al. in U.S. Pat. No. 5,002,349, the liquid-crystal optical switch described by Patel et al. in U.S. Pat. Nos. 5,414,540 and 5,414,541, or the mechanically selected optical switch commercially available from JDS Fitel and described by Iqbal et al. in "High performance optical switches for multiwavelength rearrangeable optical networks," Government Microelectronic Circuits Application Conference (GOMAC) '94, San Diego, Calif., November 1994, 3 pp. Any of these switches when inserted in the WDM optical path can extract any selected combination of the WDM signals on the path. These afore described WADMs are all optical switches in the sense that they involve no conversion from the optical to the electronic domain in performing the switching. They are referred to as transparent because an optical signal incident upon the such a switching circuit is transmitted in the same form on the output of the switching circuit regardless of the format of the signal. These optical switches of Cheung et al. and Patel et al., however, are still considered immature technologies or not suited for commercial use.
Yoo has suggested an elegant component for the wavelength-interchanging cross-connect in U.S. Patent 5,434,700. Bhat et al. describe improvements to the device in U.S. patent application Ser. No. 08/602,391, filed Feb. 16, 1996, which has issued as U.S. Pat. No. 5,802,232 and has been published as PCT document be WO-97/29,999 on Aug. 21, 1997. Antoniades et al. describe other improvements directed to the network implementation in U.S. patent application Ser. No. 08/568,037, filed Dec. 6, 1995, which has issued as U.S. Pat. No. 5,825,517 and has been published as PCT document WO-97/21,289 on Jun. 12, 1997. The Yoo device is an all-optical wavelength converter based upon non-linear optical interactions in a quasi-phase matching semiconductor waveguide.
Yoo's wavelength converter, and the network implementations of Antoniades et al. are advantageous because there is no need to convert the optical signals to the electrical domain to achieve the wavelength conversion. In a sense, their WIXC extends the transparency of transmission across multiple networks because no opto-electronic conversions are required in the multiple network transmission, thus eliminating any format dependency in the conversion, but changes in wavelengths are permitted. Opto-electronic conversions are disadvantageous in a network because they depend upon bit-rate and format.
However, Yoo's wavelength converter and its network implementation also represent relatively new and complex technology. It is desired to rely on more conventional technology for major parts of the existing telephone network which will be only incrementally and incompletely changed to WDM.
Furthermore, a cross-connect between two WDM networks will likely be placed at a pre-existing major central office or switching hub, either of which has a large number of conventional electronic or optical SONET links that need to be switched into or out of the WDM networks. Furthermore, supervisory and maintenance signals need to dropped or added at the cross-connect. A high-speed digital cross-connect switch (SCS), which typically provides such switching and drop/adding, can be connected to an all-optical WIXC as an immediately adjacent node, but at the cost of repetitive switching.